EATS 1011 3.0 Introduction to Atmospheric Science 3 Laboratory: Solar Flux and Albedo OBJECTIVES zTo measure the intensity of the solar flux, or cloud properties. zTo measure the albedo of various materials around us. REQUIRED EQUIPMENT zPyranometer and voltmeter (to measure radiation) zA watch (for the time of day) zTable of Solar Zenith Angle vs Time (to be provided by Glenn) zSamples for albedo measurements zSpotlights for indoor measurements 3.1 Introduction The intensity or flux of solar radiation crossing unit area at the Earth's orbit (1 A.U. from the Sun) is called the solar constant and will be denoted by.is the radiation striking unit area if the rays are normal (or perpendicular) to the surface. However, if the rays are not normal to the surface, then they are spread out over a greater effective area. If we define the angle θbetween the normal to the surface and the direction to the light source (i.e. sun), then the flux is diluted by the factor cos(θ) as shown in Figure 1. If the ground is flat and level then the angle theta is just the solar zenith angle (SZA), i.e. the angle between the zenith (directly overhead) and the direction to the sun. ⊕E⊕ESolar radiation enters the atmosphere as a plane-parallel beam as shown in Figure 2. Some of the light passes through unattenuated such as ray 4 in Figure 2 while some scatters off air molecules (labeled M) and aerosols (particles, labeled P) as shown by rays 3 and 2 respectively. Depending on the weather some radiation may be scattered off or be transmitted by clouds as illustrated by rays 1 and 5. Finally, some of the radiation that reaches the surface and is reflected may itself be scattered back down as shown by ray 6. Of these various types of rays we identify those that are unscattered, such as ray 4, as the direct beam, while the remainder we call the scattered radiation.1

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Figure 1: Solar flux incident on a surface at a SZA of θ. Notice that the flux at the surface is cos(θ). ⊕EFigure 2: Scattering of solar radiation within the atmosphere 2

The albedo, A, of a material is the fraction of light incident that is reflected from a surface It can be a function of the angle of incidence of the radiation, the angle at which the reflected radiation is measured, and the wavelength. You have seen during the lectures that the albedo of a planet is critical in terms of the amount of energy that is absorbed from the incident solar flux. For our Earth at least half the energy is reflected from clouds while the remaining is reflected from water, plants, soil etc. As the surface of the planet changes due to deforestation, changing farming methods, changing climate (desertification, cloud or ice/snow coverage), etc. it is important to know how the albedo of the planet may change.

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